call_user_func_array() expects parameter 1 to be a valid callback, function 'add_your_fields_meta_box' not found or invalid function name code example

Example: Warning: call_user_func_array() expects parameter 1 to be a valid callback

Somewhere in your theme or plugins is a line like this:

add_filter( 'something', 'regis_options' );
Could also be add_action(). Find that piece of code and remove or fix it.